The present invention relates a digital video tape recorder (hereinafter referred to as digital VTR) for recording digital video and audio signals on respective predefined areas on oblique tracks, and in particular to a magnetic recording and playback device into which the digital video signal and audio signal are input in the form of a bitstream, and recorded, and played back.
FIG. 64 shows tracks in a conventional consumer digital VTR. As illustrated, a magnetic tape 11 is provided with a plurality of oblique tracks in the head scanning direction inclined with respect to the tape transport direction, and digital video and audio signals are recorded on the tracks. Each oblique track is divided into a video area for recording digital video signals and an audio area for recording digital audio signals.
There are two methods for recording video and audio signals on the video tape of such a consumer digital VTR. In one method, analog video and audio signals are input, and a high-efficiency encodes is used to record the video and audio signal. The other method is a transparent recording method in which the digitally transmitted bitstream is recorded.
For recording ATV (advanced television) signals being discussed in the United States, the latter transparent recording method is considered suitable. The reason is that the ATV signal is a digital compressed signal, and does not require a high-efficiency encoder and decoder, and there is no degradation in the picture quality as the data is recorded as it is. A drawback of the transparent recording is the poor picture quality during special playback, such as fast playback, still playback and slow playback. In particular, if the bitstream is recorded on the oblique tracks as it is, almost no picture is reproduced during fast playback.
An improvement of the picture quality in the digital VTR for recording the ATV signal is proposed in an International Workshop on HDTV '93 concerning the HDTV held in Ottawa, Canada, on Oct. 26 to 28, 1993, in an article "A Recording Method of ATV data on a Consumer Digital VCR." The contents of this proposal will be described next.
As a basic specification for the prototype consumer digital VTR, the recording rate is set to be 25 Mbps and the field frequency is 60 Hz in the SD (standard definition) mode, one frame of image is recorded over 10 tracks. If the ATV signal data rate is 17 to 18 Mbps, transparent recording of the ATV signal in the SD mode is possible.
FIG. 65A and FIG. 65B show tracks formed on a magnetic tape in a digital VTR. FIG. 65A shows the scanning traces of the rotary head during normal playback, while FIG. 65B shows a scanning trace of the rotary head during fast playback. It is assumed that the rotary heads are provided in opposition, 180.degree. apart, on a rotary drum, and the magnetic tape is wrapped around the drum over 180.degree.. In the drawing, the tracks A and tracks B are scanned by rotary heads having different azimuth angles, and are formed alternately. During normal playback, the tape transport speed is the same as in recording. The rotary heads therefore trace along the recorded oblique tracks as indicated by arrows in FIG. 65A. During fast playback, however, the tape speed is different from that for recording, so that the rotary heads cross several tracks on the magnetic tape 11 during tracing, and signals are reproduced only from fractions of the tracks of identical azimuth. FIG. 65B indicates the region from which the signals are reproduced during five-time speed playback by an arrow having a width corresponding to the track width. That is, only fractions of digital data can be reproduced from the three regions which are marked with dots.
With the bitstream in accordance with the MPEG2 (Moving Pictures Expert Group 2) standard, only the intra-encoded blocks are decoded independently, i.e., without referring to other frames. If the MPEG2 bitstream were recorded on each track in the order corresponding to the order of frames and the position on the display screen, the picture would have to be reconstructed from the intra-codes obtained from the burst (part of the signal from the head having a sufficient amplitude) of the playback data during fast playback. The areas on the screen that would be reproduced would not be continuous, and fractions of blocks would be scattered on the screen. Moreover, since the bitstream is variable-length encoded, there is no guarantee that the entire screen is periodically updated, and the playback data for a part of image area may be left un-updated for a considerable time. As a result, the picture quality during fast playback is not satisfactory, and is not acceptable for a consumer digital VTR.
FIG. 66 is a block diagram showing the configuration of the recording section of the bitstream recording and playback device capable of fast playback. Reference numeral 1 denotes an input terminal, 2 denotes a variable-length decoder, 3 denotes counter, 4 denotes a data extractor, 5 denotes an EOB (end of block) appending circuit, 6 denotes a timing signal generator for generating sync signal and timing signals including a signal indicating the track on which recording is to be made, and 7 denotes a format circuit. The format circuit 7 constructs sync blocks of the recording signals, on the basis of the timing signal from the timing signal generator 6, and forms recording signals so as to record the duplication area data from the EOB appending circuit 5 on predefined track positions. Reference numeral 8 denotes a digital modulator, which performs digital modulation on the basis of the timing signal from the timing signal generator 6, appending the pilot signals for use in tracking during playback, to the recording signals for each track. Reference numeral 9 denotes a recording amplifier, 10a and 10b denote heads A and B having different azimuths A and B, and 11 denotes a tape.
The image area on each track is divided into main areas for recording bitstreams of all the ATV signal, and duplication areas for recording important parts (HP data) of the bitstream used for reconstruction of pictures during fast playback. During fast playback, only the intra-encoded blocks are effective, so that only the intra-encoded blocks are recorded as HP data in the duplication areas. But, for reducing the amount of data further, low-frequency parts of the intra-encoded blocks are extracted and recorded as HP data.
The recording operation of the bitstream recording and playback device of the above configuration will next be described. The MPEG2 bitstream is input via the input terminal 1 directly to the format circuit 7, where it is constructed into a recording signal in accordance with the timing signals from the timing signal generator 6. The recording signal is then supplied to the digital modulator 8, which performs digital-modulation of the recording signal in accordance with the 24-25 modulation system for consumer digital VTRs. The pilot signals for use in tracking are also appended at the time of the digital modulation. The pilot signals include signals having frequencies different between adjacent tracks.
FIG. 67 shows an example of a pattern of tracking pilot signals in a consumer digital VTR. As the pilot signals for tracking, pilot signals of frequencies f1 and f2 are alternately recorded in B azimuth tracks recorded by the head 10b, while no pilot signal is appended to A azimuth tracks, (which are called f0 tracks). The regions y1 and y2 are parts of the trace by the head A exceeding the width of the recording track (track pitch) on the magnetic tape 11. The areas of these regions represent the magnitudes of the cross-talk components read as f1 and f2 pilot signals from the tracks adjacent to, and to the left and right of the f0 track. The frequency spectra of the playback signals from the A and B tracks are as shown in FIG. 68. The recording signal to which the pilot signals are appended is amplified at the recording amplifier 9, and then sequentially recorded by the heads 10a and 10b in the main areas on the respective tracks on the tape 11.
The bitstream via the input terminal 1 is also input to the variable-length decoder 2, where the syntax of the MPEG2 bitstream is analyzed, and intra-encoded blocks are thereby detected. The counter 3 generates a timing signal from this intra-picture, and the data extractor 4 extracts the low-frequency components of all the blocks, and the EOB appending circuit 5 appends EOBs. The output of the EOB appending circuit 5 is supplied to the format circuit 7, which forms HP data. In the same way as the recording data for the main areas, the format circuit 7 forms recording signal for recording in the predefined positions on the recording tracks in accordance with the timing signals from the timing signal generator 6, and this recording signal is supplied to the digital modulator 8. The digital modulator performs digital-modulation of the recording signal, and the recording amplifier 9 amplifies the recording signal, and the amplified recording signal is then recorded by the heads 10a and 10b in the duplication areas on the respective tracks on the tape 11.
FIG. 69 is a block diagram showing the configuration of the playback section of the conventional bitstream recording and playback device. In the drawing, 10a and 10b denote heads, 11 denotes a tape, 12a and 12b denote playback amplifiers, and 13 denotes a switch for switching between the outputs from the A azimuth tracks (f0 tracks in FIG. 67) and B azimuth track (f1 and f2 tracks in FIG. 67) in accordance with the head selection signal indicating the head which is reading the signal. Reference numeral 14 denotes a discriminator for discriminating the signal, i.e., finding whether the signal read by the head represents "1" or "0". Reference numeral 15 denotes a digital demodulator, 16 denotes a data separator, 17 and 18 denote output terminals, 19 denotes a bandpass filter for extracting pilot signal component of frequency f1 in the playback signal, 20 denotes a bandpass filter for extracting pilot signal component of frequency f2 in the playback signal, and 21 and 22 denote envelope detectors for envelope-detect the outputs of the bandpass filters 19 and 20. Reference numerals 23 and 24 denote sample-hold circuits, 25 denotes a timing signal generator, 26 denotes an error detector which detects the difference between the cross-talk components of the pilot signals of frequencies f1 and f2 in the playback signal, and thereby detects the tracking error, and 27 denotes a servo circuit for performing tracking control and the like.
During normal playback, the heads 10a and 10b read the playback signal from the tracks on the tape 11, and the playback amplifiers 12a and 12b amplify the signals, and supply them to the switch 13, which alternately selects the signals from the heads in accordance with the head selection signal from the timing signal generator 25. The discriminator 14 discriminates the playback signal, and the digital demodulator 15 performs digital demodulation to restore the original bitstream, which is then supplied to the data separator 16. The data separator 16 separates the bitstream into the bitstream recorded in the main areas and the HP data in the duplication areas, and supplies the bitstream as normal playback data via the output terminal 17, and the data of the duplication areas as the fast playback data via the output terminal 18, to the MPEG2 decoder provided outside the playback system of the digital VTR.
During normal playback, the HP data is discarded. The output of the playback amplifier 12a, which is the playback signal read by the head 10a from the A azimuth tracks is also input to the bandpass filters 19 and 20, which extract the f1 and f2 components in the playback signal. The respective frequency components from the bandpass filters 19 and 20 are envelope-detected at the envelope detectors 21 and 22, and then supplied to the sample-hold circuits 23 and 24.
The sample-hold circuits 23 and 24 sample and hold the f1 and f2 components having been envelope-detected at the envelope detectors 21 and 22 in accordance with the sampling pulses from the timing signal generator 25, and supply the values at the sampling points to the error detector 26. The error detector 26 detects the difference of the f1 and f2 pilot signal components, to thereby detect the tracking error, and supplies the result of detection to the servo circuit 27. The servo circuit 27 effects tracking control in accordance with the result of error detection.
FIG. 70A shows the recording format used by the conventional bitstream recording and playback device, and FIG. 70B shows the sampling pulse used for tracking error detection. The sampling pulse supplied from the timing signal generator 25 to the sample-hold circuits 23 and 24 is positioned at the lower end of the track, and has a sampling point in an ITI area to which a pilot signal of a constant amplitude is appended. Then, a tracking control signal is formed from the result of error detection at the above-mentioned sampling point.
FIG. 71A and FIG. 71B explain the relationship between the tracking error signal (TC) and the servo control. FIG. 71A shows the state in which the tracking error is zero, while FIG. 71B shows the state in which there is some tracking error. When the head A is correctly tracking the A azimuth track, the head A reads the playback signal from the f0 track, while following the scanning trace S0, and the f1 and f2 components y10 and y20 of the pilot signals (PLS) which are the cross-talk components from the left and right, adjacent tracks are of the same magnitude. When there is a tracking error x as shown in FIG. 71B, and the scanning trace is S1, the difference between y11 and y21 is no-zero, and its sign depends on the direction of the error.
Accordingly, the pilot signal frequency f1 and f2 components y1 and y2 contained in the playback signal from the head A during playback are extracted at the sampling point (SPN), and the correction is made such that the magnitudes of these components are equal. That is, the error detector 26 detects the difference (y1-y2) between y1 and y2, and tracking control is made such that (y1-y2) equals zero. In this way the tracking error x is reduced to zero.
During fast playback, the tracking control is performed at the ITI area on the track, in the same way as in normal playback described above, and the playback signals (PBS) from the heads 10a and 10b are amplified at the playback amplifiers 12a and 12b. The switch 13 selects the playback signals and supplies them to the discriminator 14. The discriminator 14 discriminates the playback signal supplied via the switch 13, and the digital demodulator 15 performs digital demodulation to restore the bitstream playback signal, which is then supplied to the data separator 16. The HP data from the duplication areas separated at the data separator 16 and output to the output terminal 18 is collected, and supplied to the decoder, while the bitstream from the main areas is discarded.
The disposition of the main areas and the duplication areas on the track will be described next.
FIG. 72A and FIG. 72B explain the fast playback. FIG. 72A shows the scanning trace of the head, and FIG. 72B shows the tracking regions from which reproduction is possible. If the tape speed is an integer multiple speed, and phase-locked control is achieved, the head scanning is in synchronism with identical azimuth tracks. Of the tracks alternately recorded by two recording heads A and B, the data recording positions from which reproduction (to any degree) by the head A is possible are fixed at the portions painted with solid black, within the region of arrow S. The width of the arrow S represents the width of the head. If the effective reproduction is possible from such a part where the output level of the playback signal (PBS) is larger than -6 dB, the data effectively reproduced by one head from the tracks A are those recorded in the regions meshed in FIG. 72B.
FIG. 72A and FIG. 72B show the case of 9-time speed playback, and at 9-time speed playback, the reading of signals from the meshed regions is ensured. However, at other speeds, signal reading is not ensured. To form a configuration which enables reading at various tape speed, the regions in which the HP data is recorded need to be selected properly.
FIG. 73 shows overlapping regions of duplication areas for three tape speeds at which the head is in synchronism with identical azimuth tracks. The scanning regions from which reading by the head is possible include overlapping regions for different tape speeds. By selecting the duplication areas from the overlapping regions, reading of HP data is ensured. The drawing shows the example where the fast playback is at 4-time, 9-time and 17-time speeds. However, the illustrated scanning regions are identical to those for -2-time, -7-time and -15-time fast playback speed (reverse playback).
The fact that there are overlapping regions for various tape speeds does not mean that it is possible to determine the recording pattern such that the same regions are always traced by the head. This is because the number of tracks crossed by the head differs depending on the tape speed. Moreover, it is necessary that the tracing by the head can be started at any identical azimuth track. A solution to this problem is to repeatedly record identical HP data on a plurality of tracks.
FIG. 74 shows an example of head traces at different tape speeds. In the illustrated example, regions 1, 2 and 3 are selected from the overlapping regions for five-time and nine-time speeds. In this way, by repeatedly recording the HP data with a period of 9 tracks, the HP data can be reproduced at either of the five-time speed and nine-time speed.
FIG. 75A and FIG. 75B show an example of a head trace at five-time speed. As will be seen from this drawing, by repeatedly recording identical HP data on the same number of tracks as the number of the multiplier of the playback speed (the ratio of the fast playback speed to the normal playback speed), the HP data can be read by the head A or head B in synchronism with the identical azimuth tracks. By providing the duplication areas on the same number of tracks as the multiplier of the maximum playback speed, and repeatedly recording HP data, the HP data can be read at various speeds, and in either the forward or reverse direction.
FIG. 76 shows the recording format on the tracks in a conventional digital VTR. Main areas (MNA) and duplication areas (DPA) are disposed on one track. In a consumer digital VTR, the video area in each track is formed of 135 sync blocks. In the illustrated example, the main areas are formed of 97 sync blocks, and the duplication areas are formed of 32 sync blocks The duplication areas are the overlapping regions for the 4-time, 9-time and 17-time speeds. The data rate of the main areas is about 17.46 Mbps. Identical data are recorded 17 times in the duplication areas, so that their data rate is about 338.8 kbps.
Because the conventional bitstream recording and playback device is configured as described above, the duplication areas for recording fast playback data (HP data) are limited to the regions from which the reproduction is commonly possible at a plurality of fast playback speeds. Moreover, recording track non-linearity may occur during recording due to the head, the drum transport mechanism, or the mounting position of the head on the drum, or when the head trace during playback is non-linear, then there occurs deviation from the servo tracking point. When the track non-linearity or the like occurs, some of the fast playback data cannot be reproduced, even if the duplication areas are disposed at a plurality of locations on the head traces.
In particular, when the fast playback data is reproduced from the duplication areas disposed at locations distant from the servo tracking control point, the effects of the track non-linearity on the magnetic tape is considerable.
A second problem is that when the fast playback data is recorded on limited regions only, taking account of the track non-linearity, the amount of data that can be recorded is reduced. This is because the duplication areas for recording fast playback data are limited by the head traces during the highest-speed fast playback.
A third problem is that it is necessary to identify the position of the playback track during fast playback. Depending on the setting of the playback speed, the duplication areas may be disposed in which the fast playback data are repeatedly recorded for a predetermined number of tracks. In such a case, the frequency pattern of the pilot signals is identified from the tracks adjacent to the track in which the duplication area is disposed, the recording signals are configured in a predefined recording format, and the positions of the playback tracks are identified on the basis of the pilot signals during fast playback.
The tracking control during fast playback is achieved by the head A of the same azimuth as the head used to record f0 tracks. As a result, the fast playback data recording regions are limited by the fast playback traces of the head having recorded the f0 tracks. Accordingly, tracking control cannot be made using the head B of the different azimuth from the head having recorded the f0 tracks.
A fourth problem is that because the fast playback data recording regions are limited and the amount of data that can be recorded is limited, it is not possible to reproduce the data without fail by the conventional tracking control.
A fifth problem is that bandpass filters for extracting the pilot signal components for the tracking control have respective delay times, and the delay times differ depending on the pilot signals. If the delay times differ, it is not possible to accurately detect the error signals from the pilot signal components, and to achieve accurate tracking control.